Bad news: More than 40 percent of all Americans still live in places with unhealthy levels of either particle or ozone pollution – a severe health threat that only gets worse as the days get hotter and longer.

Air pollutants – particularly the microscopic particle pollution from car exhaust, factories and aerosols – can cause lung and heart disease and even premature death. Particles in particular damage your heart by causing a premature hardening of the arteries, a key risk factor for heart disease, heart attack and stroke, according to a recent study.

Air pollution can be so harmful for children that German researchers recently likened its health effects to secondhand cigarette smoke, saying traffic pollution alone is enough to cause asthma in children.

Although pollution harms children, seniors and people with preexisting lung or heart conditions the most, even healthy, able-bodied adults can suffer. One example: Urban runners who take their workouts outside have more dangerous brain inflammation than rural runners and even score lower on IQ tests, Belgian researchers found last year, saying particle pollution was to blame.

Wondering how your city’s air stacks up? Take a deep breath and check out the slideshow above to find out.